Valves for reversing the fluid flow direction in a fluid system are generally known and take many forms. When the highest system pressure differs substantially from the lowest system pressure, the system flow reversing valve is subjected to difficult operating conditions. Such conditions place limitations on the types and kinds of reversing valves employed. Refrigerant reversing valves used in vapor compression refrigeration systems are exemplary.
Refrigerant flow reversing valves are typically placed in refrigeration systems, such as heat pumps. The high refrigerant pressure in the system may exceed the system low pressure by more than 400 psi. This pressure differential is applied to the reversing valve structure so that it is subjected to significant pressure forces. These forces contribute to the possibility of refrigerant leakage from the high pressure side to the low pressure side within the valve as well as creating frictional forces resisting valve actuation.
The prior art proposed a reversing valve construction which was successful in reducing leakage by relying on differential pressure forces to assure sealing relationships between valve parts. This type of valve construction is illustrated by U.S. Pat. No. 4,712,582 issued Dec. 15, 1987 to Marks (the '582 patent). This type of valve relied on a valve slide member in a high pressure chamber coacting with alternative pairs of ports in a valve seat to control the system refrigerant flow direction. The slide member moved along the seat from one flow directing position to an alternate, flow reversing position. High pressure system fluid forced the slide member against the seat at all times during compressor operation. The valve seating force tended to minimize refrigerant leakage from the high pressure system side to the low pressure side. Low leakage rates improved system performance and efficiency.
The seating forces produced great frictional forces opposing valve slide member movement. Accordingly in some prior art systems the reversing valve was operated only after the compressor had ceased operation and the system pressures had been permitted to equalize or move toward equalization. In systems where flow reversal was required while the compressor operated, powerful actuators were required to overcome the frictional forces and operate the reversing valve. The control and construction of reversing valve actuators significantly complicated the reversing valve assemblies while adding materially to their cost.
A number of actuator concepts were proposed over time. The '582 patent actuator, for example, comprised a pilot valve assembly and actuating pistons mounted to the slide member and confronting respective opposite ends of the cylindrical high pressure chamber. The pilot valve assembly controlled the communication of system refrigerant to the valve chamber.
The pilot valve assembly was formed by a low power control solenoid and a miniature four way pilot valve operated by the solenoid. When the solenoid was deenergized the pilot valve was conditioned to simultaneously supply high pressure refrigerant to one chamber end and low pressure refrigerant to the other chamber end. The pistons were subjected to differential refrigerant pressure force sufficient to shift the slide member to one of its positions.
Energizing the solenoid actuated the pilot valve to reverse the pressure application to the pistons. Whenever the piston pressures reversed, the slide member was driven to its alternate position on the valve seat and system refrigerant flow direction reversed.
Such actuators, while effective and reliable, consisted of numerous parts requiring many manufacturing operations to fabricate. These kinds of valve constructions did not lend themselves to automated production. Reversing valve costs were high because of their relatively large labor content.
Furthermore, different sized valves were required for different sized systems. Refrigeration systems built to chill small, relatively well insulated spaces contain small amounts of refrigerant circulated at low flow rates through relatively small heat exchangers. High capacity systems for chilling large, or uninsulated, spaces contain much larger amounts of refrigerant circulated at high flow rates through large heat exchangers. A reversing valve, of the sort referred to, constructed for a high capacity system was capable of handling the refrigerant flow of a smaller system; however, small systems could not be relied upon to produce sufficient energy to properly actuate an oversized reversing valves.
Where a small capacity system was provided with an oversized valve, the valve could fail to complete a flow reversal if the reversal occurred when the compressor was off. Actuating such a valve about half-way through its stroke resulted in the valve member communicating the system high and low pressure sides with each other. Large area flow passages provided by the valve member rapidly depleted and diverted the supply of high pressure refrigerant which would otherwise be available for actuating the valve member through the remainder of its stroke. Accordingly the valve member could remain in its partially actuated condition even after the compressor started up again. The possibility of such occurrences necessitated constructing valves which were sized to accommodate system flows as well as to assure adequate valve actuating pressure differentials.
Modern refrigeration system components have also created conditions which have adversely impacted reversing valves. Unlike previous compressors, the newer scroll type refrigerant compressors were positive displacement compressors which could "pump" liquified refrigerant from within the compressor to the system. Scroll compressors may discharge liquified refrigerant into the high pressure side of the system via the reversing valve for an appreciable time after compressor start up. This occasionally created reversing valve problems.
When the valve slide member is between its alternative operating positions, high pressure refrigerant flow from the chamber could be restricted significantly. If this valve condition existed while a scroll compressor was pumping liquified refrigerant into the reversing valve chamber, an extreme pressure spike could be created within the valve chamber itself. Such pressure spikes could damage or destroy the reversing valve.
To lessen the great frictional forces while operating a reversing valve, a proposal was made to construct a manually operated reversing valve having a spring biased venting valve associated with it. See U.S. Pat. No. 2,855,000, issued Oct. 7, 1958 to Van Allen, et aL The venting valve was manually opened against the spring bias to reduce the pressure differential applied across the reversing valve member. This reduced the magnitude of the frictional force resisting reversing valve member movement. The reversing valve member was subjected to rapidly changing differential pressure forces causing it to dither and exhibit somewhat reduced frictional force opposing its movement.
The reversing valve was then manually actuated to reverse the flow direction. After the reversing valve was operated, the biasing spring reclosed the venting valve. This proposal was not commercially feasible in an environment requiring automatic flow reversals because the required manual operation could not be duplicated by simple, low force mechanical actuators.
The present invention provides a new and improved valve and method for reversing fluid flow circulating in a fluid system which is so constructed and arranged that the valve member is relieved from system fluid pressure forcing it against its seat as it moves between alternative flow directing positions along a path of travel by which it is separated from its seat, thereby avoiding substantial friction forces opposing valve member motion and enabling use of simple, low force valve actuators. The new reversing valve is of uncomplicated construction, comprises relatively few, easily fabricated and assembled components and yet is reliable and effective in operation.